Membrane technology usinng ethyl cellulose, acetyl cellulose or silicon-carbonate copolymer membranes to separate a specific component gas from a gaseous mixture containing the same has been the subjects of extensive research. These membranes are made of solid materials and are hence referred to as solid membranes. In order to separate the particular gas component in an economical and efficient manner, two requirements must be met: (1) a membrane material that permits the selective passage of the gas component to be separated must be selected; and (2) the membrane must be made as thin as possible. Most of the materials known today for use in solid membranes have low gas selectivities and those materials which have relatively high selectivities are only capable of very low rates of gas permeation.
Enhanced transport of a specific gas can be realized by using a liquid membrane of a material having a high degree of affinity for the gas to be separated. As described in U.S. Pat. Nos. 3,865,890, 3,951,621, 4,015,953 and 4,060,566, ethylene can be selectively concentrated from a mixture of methane, ethane and ethylene using a nylon-6.6 membrane impregnated with an aqueous solution of AgNO.sub.3. The membrane disclosed in these patents is rendered hydrophilic by incorporation of a hydrophilic polymer such as polyvinyl alcohol, but the life of the membrane is not very long since water used as the solvent for the aqueous solution of Ag ions will unavoidably evaporate during the use of the membrane.
U.S. Pat. Nos. 3,396,510, 3,819,806 and 4,119,408 show that acidic gas components such as CO.sub.2, H.sub.2 S and SO.sub.2 can be selectively permeated through a polyethersulfone membrane impregnated with an aqueous solution of K.sub.2 CO.sub.3. But this membrane has the same problem as in the case of ethylene separation discussed above.
European Patent Application No. 98731/1984 shows that oxygen can be selectively separated from air using a nylon-6.6 membrane impregnated with a transition metal complex of a Schiff base dissolved in a solvent such as lactone or amide. In the Example therein, a nylon-6.6 membrane with a thickness of 130 .mu.m was used and this suggests the presence of a liquid membrane that was at least 130 .mu.m thick.
This membrane system is substantially free from the problem of solvent evaporation because the liquid barrier is made not of water, but of organic solvents such as lactone and amide. However, the life of the system is short because the transition metal complex of a Schiff base is irreversibly oxidized during operation. In addition, the liquid barrier that is impregnated into the nylon-6.6 membrane cannot be made thinner than 130 .mu.m, preferably as thin as a few micrometers.